The wheels formed of lightweight metal that are preferably used in passenger automobiles greatly contribute for decreasing load underneath the springs and for improving maneuverability and fuel efficiency or mileage. Such wheels are produced by casting or forging as to provide ones having excellent ornamental appearances. In response to quest for wheels more lightweight and more excellent in the ornamental appearances, the lightweight metal wheels allegedly have a share exceeding 60% in wheels for the passenger automobiles. With increase of speed capacity of the passenger automobile, there is required enlargement of brake mechanism and of the wheel; thereby, diameter of the wheels increases year by year and may well soon exceed 17 inch in nowadays trend.
In producing the lightweight metal wheels, aluminum or magnesium is mainly used as material for the wheel, and the forging is preferably used in order for achieving densely packed metallographic structure and thereby for enhancing stiffness of the metal.
Method of the forging in prior art is exemplarily shown by JP-1985(S60)-127040A (Prior-Art Patent Document 1), which shows a method comprising of; a step of forging a disc-shaped work having thick-wall outer fringes; a step of forge-wise forming, from the disc-shaped work, a disk part having edges in predetermined shape and relief-wise formed predetermined window openings as recesses, or such disk part and a ring part integrally continuous with the disk part; a step of cutting out bottoms of the recesses as to form window openings; and a step of chamfering inner peripheral fringe of the window openings.
If diameter of the disk is less than about 18 inch, a press device having capacity of about 8000 metric ton is enough for pressing whole face of the disc. If the diameter of the disk exceeds the 18 inch, it is difficult to press the whole face of the disk by use of the press device of about 8000 metric ton capacity and thereby press device of about 10,000 metric ton capacity is needed.
Portion-by-portion stepwise forging technique has been known as for forging a member having large area by use of a small-scale press device. As for forming a disk of simple shape by such technique, JP-1983(S58)-12092A (Prior-Art Patent Document 2) for example shows a method for forging a beam flange for warping of yarns; in which a starting metal piece is portion-by-portion stepwise pressed as to omit a need of large-scale pressing device; and in which then obtained disk is progressively enlarged by properly changing upper mold. By this method, the flange having diameter of 30 inch or more may be obtained, while no “pattern” may be formed because the disk is drawn not only in a circumferential direction but also in a radial direction on course of the stepwise pressing.
Other than the portion-by-portion stepwise forging technique, rotation forging technique has been known and proposed by numerous prior-art documents. In principle, a tool having conical face is tilted and abutted on a work and the forming is gradually and successively made in circumferential direction by such way of pressing. Typical methods in the rotation forging technique are shown in JP-1994(H06)-154932A (Prior-Art Patent Document 3) and JP-1994(H06)-285575A (Prior-Art Patent Document 4).
In the method shown in JP-1994(H06)-154932A, a work that has a to-be-disk part and a to-be-rim part protruded in a ring shape from periphery of the disk part is prepared; and then the work is rotated and at same time compressed by a lower tool for processing outer face of the to-be-disk part and an upper tool for processing inner face of the to-be-disk part, while the to-be-rim part is abutted by a forming roller, as to form an integral wheel. In the method shown in JP-1994(H06)-285575A, a set of upper and lower tools in which concave and convex portions are alternately formed along a circle line with a constant amplitude is used; and firstly, one of the concave portions is aligned with one of the convex portions and then pressing is made, and such forge processing is made along a whole circle by rotating the tools as from such firstly aligned position. In such rotation forging technique, material of the work is migrated in radially outward directions. Thus, a disk pattern stretched in a radial direction may be formed whereas more complicated “pattern” is not achievable.